The invention relates to a sensor for measuring the flow velocity of gases and to a method for the production thereof.
It is known, for example from DE 32 34 146 A1, DE 196 23 323 A1 or DE 27 23 809 A1, to measure the velocity of gas flows thermally on the basis of the xe2x80x9ctime of flightxe2x80x9d. A transmission wire arranged in the gas stream transmits a periodic heat signal. The heat signal is transported by the flow and is sensed after a xe2x80x9ctime of flightxe2x80x9d by a reception wire arranged downstream. The xe2x80x9ctime of flightxe2x80x9d or the phase shift proportional to the latter of the heating signal sensed by the reception wire is a measure of the flow velocity.
In the xe2x80x9ctime-of-flight measuring methodsxe2x80x9d, it is assumed that the relationship between the time of flight xcex94t and the flow velocity U is linear. The following relationship applies for calculating the flow velocity
xcex94t=xcex94x(1/U),
where xcex94x is the distance between the transmission wire and the reception wire.
A sensor known from the brochure xe2x80x9cProbes for hot-wire anemometryxe2x80x9d of the company Dantec Measurement Technology A/S, Denmark, has a transmission wire and reception wire, which are respectively mounted on the tips of two prongs protruding into the gas stream in order to avoid heat losses. The mounting of the transmission wire and reception wire on the tips of the prongs is difficult. In the case of such sensors, the distance between the transmission wire and the reception wire can scarcely be set in a reproducible manner. They must therefore be calibrated before they are used. Each sensor has a specific calibration curve. The evaluation electronics must be re-tuned to each sensor.
Further sensors are known from DE 196 09 823 A1 and DE 28 45 661 C2. The production and/or mounting of these sensors is complex.
It is an object of the invention to specify a sensor for the thermal measurement of flow velocity which can be produced in a simple and low-cost way. According to a further aim of the invention, the sensor is to be reproducible with high accuracy and, as far as possible, not have to be individually calibrated.
This object is achieved by the features of the independent claims. Expedient configurations are provided by the features of the dependent claims.
According to the invention, a sensor for measuring the flow velocity of gases is provided, a printed-circuit board provided with a clearance having two opposing first contact elements on its upper side close to the edge of the clearance and two opposing second contact elements on its underside close to the edge of the clearance, and a first wire, spanning the clearance, being connected to the first contact elements and a second wire, spanning the clearance, being connected to the second contact elements.
The sensor can be produced in a simple and low-cost way. The time-consuming and difficult mounting of the transmission wire or first wire and of the reception wire or second wire on the tips of the prongs is no longer needed. The sensor can be produced automatically and at low cost. The distance between the two wires can be reproduced with high accuracy. It is possible to dispense with individual calibration of sensors produced in such a way.
The first wire and/or second wire expediently has a diameter of 5-25 xcexcm, preferably of 12.5 xcexcm. Such a wire can be electrically bonded in an automated manner.
According to one configurational feature, the contact elements have ring segments which segmentally surround the clearance and advantageously have two first grooves, oriented parallel to each other. They may have two further grooves, which form an angle of 25-45xc2x0, preferably of 35xc2x0, with the first grooves. With the proposed embodiment, it is particularly easy for the wire to be mounted with a predetermined tensile stress. Consequently, a straightness of the wire required for high measuring accuracy is achieved. The tensile stress can be maintained in particular by the two grooves arranged at an angle, because the wire is in this case held with friction-locking support.
The contact surfaces bounding the two grooves are expediently connected in each case to one of the ring segments via a first conductor track. Each of the contact elements may be connected to a contact pad via a second conductor track. The contact pads may have clearances for wires to be electrically bonded. The proposed structuring of the conductor tracks makes it possible in practice for the sensors according to the invention to be produced in a simple procedure and without any defects.
It has proven to be expedient for the printed-circuit board to have a length of 17-23 mm, preferably of 20 mm, a width of 5-10 mm, preferably of 7 mm, and a thickness of 1-3 mm, preferably of 1.5 mm. The clearance may have a diameter of 2-5 mm, preferably of 3 mm. The distance between the first wire and the second wire is expediently less than 3 mm, preferably approximately 1.5 mm. A sensor with the aforementioned dimensions is suitable universally for measuring the velocity of a gas flow.
The second contact elements may be formed mirror-invertedly in relation to the first contact elements. In this connection, the major middle plane of the printed-circuit board is regarded as the mirror surface.
A conically widening portion of tube may be provided upstream and/or downstream of the clearance. The portion of tube preferably adjoins the printed-circuit board directly, its smaller diameter facing the printed-circuit board. The conical shape of the tube portions has the effect of accelerating the gas stream. Consequently, even very low gas flow velocities can also be measured. Furthermore, a sensor designed in such a way is also suitable for the exact measurement of small volumetric flows. Finally, the conical tube portions serve for protecting the wires from mechanical damage.
According to a further measure of the invention, a method for the production of the sensor according to the invention is provided, the first wire being placed into the first and second grooves of the first contact element, subjected to a predetermined tensile stress and subsequently electrically bonded, and the second wire being placed into the first and second grooves of the second contact element, subjected to a predetermined tensile stress and subsequently electrically bonded.
It goes without saying that it is also possible to provide a plurality of first and second grooves arranged in parallel next to one another on the first and second contact elements for receiving a plurality of first and second wires arranged next to one another.
Such arrangements may be expedient for particular measuring requirements.
One alternative provides a method for the production of a sensor according to the invention in which
a) a multiplicity of first contact elements (3) are produced next to one another on an upper side and a multiplicity of second contact elements are produced on an underside of a printed-circuit board,
b) a first wire (4) is placed over the multiplicity of first contact elements (3) under the effect of a predetermined tensile stress,
c) subsequently electrically bonded,
d) a second wire (10) is placed over the multiplicity of second contact elements under the effect of a predetermined tensile stress and
e) is subsequently electrically bonded.
In this variant of the method as well, the first and/or second wire may be placed into first and possibly second grooves formed in the first and/or second contact element. This makes it easier for an applied tensile stress to be maintained.
In the case of both variants of the method, it is advantageous to solder the first wire and the second wire to the first and second contact elements, respectively. The printed-circuit board may be turned through 180xc2x0 after the electrical bonding of the first wire and then the second wire is brought into contact with the second contact elements. As a result, the production costs can be further reduced, because only one device is required for the feeding in of wire and its electrical bonding. Here, the provision of first and second grooves is possible, but not absolutely necessary.
In the case of the second variant of the method, the printed-circuit board is expediently divided into individual boards after the electrical bonding of the first and second wires. The dividing of the printed-circuit board into the individual boards may take place by breaking at predetermined breaking points or by cutting by means of suitable cutting devices.
The proposed methods can be automated in a simple manner. High numbers of units can be produced at low cost.